1. Field of the Invention
The present invention relates to a system for detecting accurately the presence or absence of a target vehicle in front of and in the same lane as a subject vehicle.
2. Description of the Prior Art
Some recent automotive vehicles are equipped with cruising control systems which hold the vehicle speed at a desired cruising speed. In addition to the above-described cruising control system, systems for controlling vehicle speed so as to track a preceding vehicle, if any, automatically at a safe inter-vehicle distance have been proposed, wherein the opening angle of a throttle valve of the engine is controlled so as to hold the vehicle at essentially the same speed as the preceding vehicle and at a predetermined inter-vehicle distance behind the preceding vehicle. Such a system is discussed in Japanese Patent Application Unexamined Open No. Sho 55-86,000.
It is necessary for a system controlling vehicle speed to measure accurately the inter-vehicle distance to the preceding vehicle in order to appropriately control the vehicle speed. For this reason, a radar unit is installed in the front end of the vehicle to measure the inter-vehicle distance.
Although inter-vehicle distance can be measured accurately by means of a single radar unit using electromagnetic waves such as a high-frequency electraic wave or laser as a measurement medium or a sonar unit using a ultrasonic waves as the medium, the inter-vehicle distance cannot normally be measured on curved roads since a reflected waves will not return to the vehicle. Furthermore, erroneous measurements may result even when reflected waves are captured since such reflected waves may be derived from unrelated vehicles in a neighboring traffic lane.
To ensure measurement while preventing false readings of the inter-vehicle distance when the vehicle is travelling on a curved road, a system for detecting obstacle in front of the vehicle has been proposed. This is exemplified by Japanese Patent Application Examined Open No. Sho 51-7892.
FIG. 1 shows this conventional system for detecting obstacles in front of the vehicle.
In this system, a swivel unit 9 swivel a radar unit 1 across the width of the vehicle according to the steering angle reported by a steering angle sensor 5 with its equibrium position along the vehicle axis. In other words, in the system shown in FIG. 1, the radar unit 1 pivots according to the angular position of the steering wheel of the vehicle as the vehicle moves along the curved road so that reflected waves can always be captured, thereby ensuring detection of the presence of the preceding vehicle and accurate measurement of the inter-vehicle distance. Therefore, a signal processing circuit 7 controls the vehicle speed via a vehicle speed acceleration/deceleration mechanism 11 in order to maintain a safe inter-vehicle distance on the basis of the results of the above-described distance measurement and of the vehicle speed detected by a vehicle speed sensor 3.
However, in this method for pivotting the radar unit 1 according to the steering angle, the actual steering angle does not always agree with a curvature of the curved road, so that the measurement medium transmitted from the radar unit may not reach the preceding vehicle.
Therefore, the task of determining the presence or absence of the preceding vehicle has not yet been fully resolved.